The present invention relates to a method for producing an electrochemical device.
The existing electrochemical device that is fueled by alcohol aqueous solution is composed of an anode electrode, a cathode electrode, and an ion exchange membrane formed from a polymeric solid electrolyte, all of which constitute the MEA (Membrane & Electrode Assembly). The ion exchange membrane is usually made of monomer such as Nafion (a registered trade mark of DuPont for perfluorosulfonic acid resin), which has a high ion conductivity. Nafion is used also as a binder in the electrode. (See, for example, Japanese Patent Laid-open No. Sho 61-67787 (from column 11, line 16, to column 15, line 7) and Japanese Patent Laid-open No. Sho 61-67788 (from column 12, line 9, to column 16, line 2.)
The fuel for the electrochemical device is dominated chiefly by methanol. Other fuels under study include hydrogen gas and organic materials such as ethanol, dimethyl ether (DME), and diethyl ether (DEE). (See, for example, Japanese Patent Publication No. Hei 3-208260, from column 3, line 1 to line 16, and FIG. 1.)
Promising among these electrochemical devices is the direct methanol fuel cell (DMFC), which uses methanol aqueous solution as a fuel for direct reaction with the anode electrode. The DMFC is highly expected to be a small portable fuel cell as a next-generation power source on account of its high energy density.
The electrochemical device such as direct methanol fuel cell mentioned above is required to have improved output efficiency and durability. However, the direct methanol fuel cell is inferior to hydrogen fuel cells in thermal durability, chemical durability, and mechanical durability.
Nafion (registered trade mark) mentioned above or conventional sulfonated fluorocarbon polymer is liable to dissolve in the alcohol fuel and hence it liberates catalyst particles due to poor binding force, resulting in a reduced output. The tendency toward dissolution causes not only the peeling of the catalyst layer but also the weakening or breakage of the MBA film.
For this reason, the binder in the electrode and the ion exchange membrane (or polymeric solid electrolyte membrane) should be formed from a material which is hardly soluble in alcohol fuel and which has a high proton conductivity.